<p>The effect of high ion temperature on the ion polytropic coefficient (γ) was experimentally investigated in the end region of the GAMMA 10/PDX tandem mirror. The ion temperature parallel to the magnetic field (T_i||) was varied using the ion cyclotron range of frequencies (ICRF) waves, which heat the plasma in the central cell, and by applying an additional gas puff in the central cell. Assuming the ion sound speed, γT_i|| was evaluated by a Langmuir probe, and T_i|| was evaluated in an ion energy analyzer. These parameters were combined to evaluate γ. The time evolutions of γT_i|| obtained from the probe and T_i|| obtained from the energy analyzer were similar. The plasma in the end region can be regarded as collisionless, but γ was lower with ICRF heating than without the heating. A high T_i|| component was identified in the heated case. As mentioned in a previous numerical study [B. Lin et al., Phys. Plasmas 23, 083508 (2016)], it is experimentally suggested that γ can be reduced by the high T_i|| component resulting from the ICRF heating.</p>